Retroviral-mediated transfer of a normal human ADA cDNA into hematopoietic stem cells of patients with ADA-deficient SCID may result in the production of mature T lymphocytes expressing ADA enzyme activity, with restoration of immune function. We will transplant ADA gene transduction of hematopoietic stem cells from patients with ADA deficiency and transplant the transduced cells into the patients. We will then determine the efficacy of ADA gene transfer and expression by serial examination of peripheral blood lymphocytes and hematopoietic cells. We will quantitate the frequency of cells which contain the ADA cDNA by semi-quantatative DNA-PCR and the level of expression from the vector by RT-linked PCR and ADA enzyme assay. Additionally, we will examine the effects of ADA gene expression on the immune function of the transplanted patients. In all, these clinical studies will determine the effectiveness of current retroviral-mediated gene transfer techniques to introduce genes into human hematopoietic stem cells, which may be used to treat a number of genetic diseases and AIDS. Additionally, the specific benefits for the immune deficiency of ADA-deficient SCID will be assessed.